Various titanium dioxide pigments in the rutile form are used as whiteners in a variety of polymeric compositions. In some such polymeric compositions, most notably polystyrene containing compositions, a chemical reaction takes place between the titanium dioxide pigment and the polymer itself which often results in the formation of undesirable compounds which impart a yellow coloration to the end product. A similar chemical reaction can take place between the titanium dioxide pigment and additives, such as phenolic antioxidants, which are used in a variety of polymers, such as polystyrene and polyethylene.
The art has recognized that the presence of anatase or hydrous titania components in the titanium dioxide pigment utilized as the whitener for a specific polymeric composition is the cause of the yellowing which often takes place.
It has also been recognized that the yellowing which takes place can be controlled by the precipitation of hydrous oxides of elements such as aluminum, silicon and zirconium upon the surface of the titanium dioxide pigment. Such treatment processes invariably involve the slurrying of the titanium dioxide pigment in water followed by filtration, drying and micronization operations in order to produce a usable titanium dioxide pigment having enhanced properties. This method of treating titanium dioxide pigments in order to control undesirable yellowing when incorporated into various polymeric compositions has the distinct disadvantage of adding a significant expense to the production of such pigments.
As an alternative, the treatment of titanium dioxide pigments with zinc stearate has been commercially employed in order to counteract the undesirable yellowing effect referred to above. However, due to the fact that zinc stearate is insoluble in most solvents deemed appropriate for the treatment of titanium dioxide pigments, this solid material is normally dry-blended with the titanium dioxide pigment, which results in an admixed end product having less than the optimum degree of coverage of the pigment by the zinc stearate treating agent, and in which some classification of the dissimilar solids occurs.
It has thus been found desirable to develop a new treatment method which avoids the disadvantages of the art employed methods described above, while affording the advantages of substantially improving control of the yellowing phenomena frequently encountered, and at the same time reducing the overall cost of producing such a treated product.
Exemplary of the state of the art as to various known treatment methods for titanium dioxide pigments are the following:
U.S. Pat. No. 3,345,187 which issued on Oct. 3, 1967 to Binnis, discloses the use of trialkanolamine phosphates, benzoates and titanates in combination, as well as trialkanolamine alone, to coat titanium dioxide pigments in order to produce corrosion resistant coatings. No mention is made of any benefits to be afforded by any such treatment relevant to color stability when such a treated pigment may be employed in a polymeric composition.
U.S. Pat. No. 3,545,994 which issued on Dec. 8, 1980 to Bott, et al. discusses the need for masking active surface sites on titanium dioxide pigments and discloses the use of homogeneous precipitation of hydrous oxides on titanium dioxide pigments in order to provide such a masking affect. This methodology is an example of a much more expensive way of accomplishing the result which is produced in accordance with the present invention at considerably less cost.
U.S. Pat. No. 4,001,034 which issued on Jan. 4, 1977 to Simkin, discloses a method of applying a coating containing titanium dioxide and a zinc phosphinate polymer to the surface of metals. This method is not directed to a treatment of titanium dioxide to reduce yellowing when incorporated in polymers.
A number of patents have issued directed to the use of phosphorus-containing organics as stabilizers for polycarbonates. Among them are the following:
U.S. Pat. No. 3,498,946 which issued on Mar. 3, 1970, directed to the use of organic phosphites;
U.S. Pat. No. 3,673,146 which issued on June 27, 1972, directed to triorganophosphite;
U.S. Pat. No. 3,729,440 which issued on Apr. 24, 1973, directed to alkyl aryl phosphinites;
U.S. Pat. No. 4,054,522 which issued on Oct. 18, 1977, directed to ester of phosphorodihalous acid;
British Pat. No. 1,347,571 which issued on Apr. 19, 1971, directed to alkyl aryl phosphonites.
None of these references are directly related to the treatment process or the product produced thereby of the present invention.
U.S. Pat. No. 3,713,859 which issued on Jan. 30, 1973 to Hoover, et al. discloses the use of amino-diphosphonates as pigment dispersants. No suggestion of any suppression of chemical reactivity of titanium dioxide pigments in order to reduce yellowing when incorporated in polymeric compositions is found in this teaching.
U.S. Pat. No. 3,904,565 which issued on Sept. 9, 1975 to Yoshiura discloses the use of a mixture of thiobisphenol, an organic phosphite, a fatty acid salt and a polyalkylene glycol ether in order to improve the anti-oxidation and lightfastness characteristics of polyolefins.
U.S. Pat. No. 4,209,430 which issued on June 24, 1980 to Weber discloses the treatment of titanium dioxide pigment with a phosphorylated polyolefin in order to inhibit phenolic yellowing in polyethylene which presents a method that is basically different from the present application, which does not utilize a treatment material produced via the phosphorylation of a polyolefin.
None of the foregoing prior art teaching suggest either the method or the product which is the subject of the present application, which involves the treatment of pyrogenic titanium dioxide pigment in order to reduce the yellowing effect which might otherwise take place when such a pigment is incorporated into a variety of polymeric compositions.
It is, therefore, an object of the present invention to provide a method for the treatment of a pyrogenic titanium dioxide pigment in order to substantially reduce the yellowing effect which might otherwise take place when such pigments are incorporated into various polymeric compositions.
It is a further object of the present invention to provide a product comprising a pyrogenic titanium dioxide pigment which has been treated in order to reduce the yellowing effect which might otherwise take place when such product is incorporated in various polymeric compositions.
These and other objects of the invention will become more apparent from the following description of the invention.